Trench cutting apparatus using high pressure water jets to form the trench are well known in the subsea environment. Until recently the primary use of such trench cutting apparatus has been in relatively deep water. However, there is increasing demand to employ trench cutting apparatus in shallower water conditions. The use of jetting tools in shallow water presents some problems.
To supply water at high pressure to the outlet(s) of a jetting tool, a pump is used to pump water from the environment around the jetting tool. Generally the pump is submerged and in relatively close proximity to the high pressure outlets. For example, the pump may conveniently be mounted to the body of an underwater trench cutting vehicle. For applications of this sort, it is preferred to use high specific speed pumps such as multi-stage axial pumps and mixed axial flow pumps since the size and weight of pumps of this type are generally lower than lower specific speed pumps such as centrifugal type pumps.
The high intake flows of high specific speed pumps have the effect of lowering the ambient fluid pressure in the intake side of the pump. In deep water conditions this is acceptable, but in shallow water conditions the reduction in ambient pressure can be sufficient to cause cavitation in the pump. As is well known, cavitation is likely to reduce the performance and service life of the pump.
One proposed solution to the problem of cavitation in shallower water is to install an auxiliary pump upstream of the primary pump, the auxiliary pump providing low pressure and high flow. Such an auxiliary pump can be mounted on the trenching vehicle or on a service vessel or on the shore, for example. However, this is inconvenient and adds complexity to the overall apparatus. It is also possible to use low specific speed pumps as the primary pump, but as noted above such pumps carry a weight penalty and are more expensive.
The present invention seeks to obviate or mitigate such problems and to provide a pump apparatus, in particular for a jetting tool of a trench cutting apparatus, which is suitable for use in shallower water and which reduces or avoids problems of cavitation.